I realize this situation is odious to you but I think I'd go, only as support to my DH.  The rehearsal dinner expense would rub me wrong, too, but unless it's a financial hardship. I'd go along.

I agree, your SS sounds like a major jerk. But I'm guessing he's at least in his thirties so yes, your DH is correct that he's a grown man.  I'm sure the karma bus will be coming for him at some point.

Long story short, I'd stay out of any discussions about him and I wouldn't contact the bride's father.  In the best case, he and the girl will make a good marriage so you want to stay on a positive note with them.  But bottom line, he's your DH's son, he will be a part of your family as long as you're with DH.  Take the long view.

Meantime, find yourself a great dress and book a spa appointment.

The bride knows he is a cheater because he was cheating on his wife with the bride. Stay as far away from this as you can. No good can come from you talking to the father or making a scene at the wedding. If you don't think you can keep your mouth shut, then don't go to the wedding.

My thoughts are as follows.

1.  Your husband is correct.. your SS is a grown man.. his fiancee. a grown woman.. she has to know he was cheating.. it's a risk she is taking.. she is special right? You don't interfere.

2.  But.. your DH is wrong to feel strong armed into paying anything for a 2nd wedding.. Personally, I feel.. that IF a parent chooses.. that the first weddings are the ones that get funded.. you don't get repeat payments for subsequent marriages.  Theoretically.. these "kids" are adults.. should be well launched.. they can pay for their own nuptials.. all of it.

Now.. if your DH chooses to give them a generous wedding gift.. money even. enough to cover that dinner.. he can do that.. it will be their choice to spend it how they wish.  I DO think you should get them a gift.

3.  I Do think his dad should go to the wedding and tongues should be held as to anyone's negative feelings about SS or the prospect of the wedding being permanent.. Go.. be pleasant.. support your DH as he supports his child.. time will tell how this all works out.

As far as other relatives choosing not to attend?  I actually think people get to be off the hook when it is your relatives 2nd.. or more.. wedding.. but I do think that parents should make an effort to go.. as long as it's not logistically difficult.

It all depends on how this rehearsal dinner affects you financially.  If it's just a bump in the road, then it's having a good time with friends and family.  If you are going to have to eat dog food to pay for this dinner it's different.   SO is doing thr father thing.  Just make sure it ends here. No marriage number 3. Crap. In the future 

As everyone has said, both people - SS and Bride - are adults. Bride is aware SS is a cheater as she is also a cheater. The rehearsal dinner payment is up to your DH. His kid, his gift (unless it puts a hardship on you guys). I would hope DH would let SS know we aren't paying for subsequent weddings. The money is a gift and you choose what to pay with it.

Me? I'd not say a word or even discuss this with anyone but my DH (trust me, people can turn on you in an instant and this could be a prime event to have such a thing happen, making you the bad guy).  I would go to the wedding, offer my congrats to the new couple, enjoy the evening and if necessary, leave early. But I would NOT let any negativity come out of my mouth. 

Not ever hill is worth dying over. This one definitely not.

Yeah, stay removed. Get your pretty dress, attend and just don't get involved whatsover. This is likely going blow up (sooner or later) and you don't want to be part of the casualities ...you also don't want to distract them from having to face their horrible behavior. I have found stepparents- specifically stepmoms become the target and distraction from terrible piss poor behavor by SKIDs- do not let them make you out to be the bad guy to diguise their crap. Let them air their own dirty laundry and remain absolutley invisible to being involved in the BS. 

I would go to the wedding if my DH wanted to go, and I'd keep my mouth shut. Your attendance is to support your DH and does not necessarily show support for the marriage. 

The bride knows SS is a cheater.  Whether she's shared that with her father is her business - it's not up to you to save him from shelling out for the wedding.  At this point it's likely water under the bridge. 

As for the rehearsal dinner, I don't think it's incumbent on your husband to pay for a rehearsal dinner.  This is the 2nd wedding for a son who has been a grown ass adult for years and should be capable of hosting his own rehearsal dinner.  If it's your DH's money, though, and this is how he wants to use it,  I'd back off.  If you have shared finances, I'd be inclined to take an equivalent amount out of the joint account and use it for something I wanted. 

I have no sympathy for the cheating bride-to-be, foolish enough to engage with a married man. As is well known, 'when a man marries his mistress he creates a vacancy'.

Hopefully, the ex-wife has come to realize that her life will improve substantially, now that Mr. Cheater is elsewhere. And, SMDb, I found out, the hard way, that negative opinions about a husband's relations should be kept to oneself.

I would. In fact, I have.  In college one of my very lose friends had been with his GF for years.  I started dating one of her coworkers.  My then GF shared that my friends GF was cheating on his with a number of their coworkers.  His GF would come back from her lunch festivities and give my GF the lowdown.   I chose to inform my friend. When I gave him the information he coroborated it from her planner. Including the initials of her daliance partners, locations, times.   He broke up with her.   He was so mad.  Not at me.

I helped him move out of their apartment.  I ended it with her coworker not long after that.

He chose to ask her out again a year or so later.  They married about the same time my DW and I did.   My DW and I went out with them a few times while we were dating. After they started dating again.  We all get along fine now. Which I find odd.  Once they started dating again, she just pretended it never happened aparently.  She was a very type A personality.  He was more of  an analytical personality type.  She has mellowed incredibly.

They seem happy. 

I would not do anything differently in hind site.  I did what I felt I had to do for my friend.

Anonomously notify the GF's father, the one paying for the wedding. See if he will still pay once he has the 411 on your SS.

Be radiant, you and DH beam your happiness together.  Have THE look ready.  So your SS knows that you know.  I would also make sure daddy is fresh on SS's cheating so he will have that in the back of his mind.  

Maybe DH has the spine and character to sit his POS son down and jerk a knot in his tail.

Wedding? Nope, relabel it as "date night" with DH.  Get all dolled up and ignore the wedding drama.  Enjoy the night off from cooking.  Leave early.   

I feel like if you reached out to FIL to be with all your pure intentions, that you would be the one that would be hated for trying to break up this wonderful couple.

So, dont sweat it, get dolled up, book a spa day and get hair nails and massage. Have your DH pay for it.

Advise your DH to just gift a set amount that is reasonable and they can budget accordingly.

* popping the corn, melting the butter * 

And please let us know how it all went!!!

And send soon-to-be ex daughter in law: 
chumplady.com

They are both adults. SS is a cheater and his future wife knows this because she was an accomplice. They are going into this with their eyes wide open. The only thing I would be concerned about is the amount your husband is willing to throw away on this wedding. He is absolutely not obliged to pay for anyone's wedding in this day and age much less their second wedding. Decide together how much you're willing and able to give as a wedding present, give the happy couple the cheque and that's it. You don't have to foot any bills you don't want to.